The disclosure of U.S. Provisional Application No. 61/552,306, filed Oct. 27, 2011, is hereby incorporated herein in its entirety by reference.
Vacuum generators that use air or another compressible gas, are well known for parts holding and pick & place applications. Within the design parameters of the vacuum generator, the maximum vacuum level attained is typically controlled by changing the inlet feed pressure of the compressed gas supply. Part release is typically obtained by turning off the inlet air supply to allow ambient air to be drawn through the exhaust nozzle to dissipate vacuum in the downstream system.
Compressible gas vacuum generators utilize a progression of gas flow nozzles for generating the vacuum. The first nozzle of a vacuum generator is configured to generate deep, maximum vacuum (greater than approximately 90% vacuum) and accomplishes this by increasing inlet air velocity to a sonic level as the feed pressure is increased and the vacuum level deepens. Until sonic velocity is approached, the induced vacuum pressure may exhibit minor low amplitude, low frequency oscillations, but is typically fairly stable overall.
Because the media is compressible gas, as deeper vacuum levels are attained, it has been observed that within a relatively narrow range of feed air pressures, random rate instability and turbulence within the vacuum generator can cause higher amplitude random rate oscillations in vacuum pressure. This period of instability is often evidenced by exhaust air noises which can be heard as rapid popping or humming or squealing noises. In aeronautical engineering literature this instability/turbulence phenomenon is well documented for aircraft as they break the sound barrier. As a rule, the vacuum generated is proportional to the velocity of the air stream in the first nozzle, and the rapid velocity oscillations have been found to be accompanied by corresponding rapid ripples and spike oscillation in the vacuum level generated, which can exceed 45 mm Hg. peak-to-trough. Because the oscillations often occur at high frequency, they do not register on a bourdon tube style vacuum gauge due to slow response time of those gauges, but can be observed with an oscilloscope.
For many industrial applications the high frequency, high amplitude vacuum oscillations are not problematic because the work pieces being held or manipulated are robust enough not to be damaged by the oscillations. Also, the attendant vacuum generator exhaust noises are often not noticed or of importance as they may be concealed by the ambient background noise of a manufacturing plant. However, high amplitude vacuum spikes can cause problems for more sensitive applications where the vacuum level must be precisely controlled, such as, but not limited to, applications such as in the medical field in which precision instruments are used and delicate parts or tissue is handled or manipulated.
Referring to FIG. 1, vacuum flow is plotted against vacuum pressure generated by a representative vacuum generator supplied by compressed air at a particular feed pressure. The resulting curve shows that the generator produces high vacuum flow at shallow vacuum levels (near atmospheric pressure) and zero or near zero vacuum or air flow, hereinafter sometimes referred to as “vacuum flow” or “air flow”, at the deepest maximum vacuum level that can be attained in a sealed system. The area under the curve from atmospheric pressure to a particular vacuum level represents the power available to evacuate the volume of a system by the vacuum generator. The curve also shows several areas where irregularities caused by turbulent air flow through the first and second nozzles of the vacuum generator affect the vacuum generation, in particular, reduce it, at about −320 mm Hg. and about −520 mm Hg.
The objectionable high amplitude vacuum pressure oscillations have been found to develop as vacuum level deepens and approaches its maximum level. This region of FIG. 1 is magnified to illustrate representative oscillations in the maximum level region, and also comparatively at a shallower vacuum region. In a vacuum generator utilizing two nozzles, this effect is believed to be due to the instability of the air stream passing through the first nozzle as the compressed air is increased to sonic or near-sonic velocity in order to generate a less-than atmospheric pressure (vacuum) in the chamber between the nozzles. The oscillations were found to be most existent near the deepest vacuum, particularly between about −620 and −690 mm Hg. and can have a peak-to-peak amplitude greater than 100 mm Hg., reaching 180 mm Hg. or so. One practical application where this has been found problematic is for supplying vacuum to certain instruments for delicate surgery of the eye which require steady vacuum in this range.
One known manner to attenuate or damp the transmission of the objectionable high amplitude vacuum pressure oscillations in a vacuum system is to use flow restrictors, such as baffles and the like. However, one shortcoming to this approach is that any flow restriction or restrictions between the nozzles and the vacuum system being evacuated by the vacuum generator will reduce the available power. Any restrictions can also cause system evacuation time to increase and will reduce the responsiveness of the overall system. Fixed flow restrictors such as baffles and the like are also disadvantageous as they are always present and thus do not modulate or vary in effectiveness in response to vacuum demand or the undesirable oscillations as they arise. For some applications such as the above referenced surgical instrument application, it is important that the oscillation damping be self-modulating to provide minimal resistance to vacuum flow throughout the full range of operation of the instruments.
Although the above description is for a single-stage vacuum generator comprising a first and second nozzle in series, it should be noted that the noted shortcomings also pertain to multi-stage vacuum generators having three or more nozzles in series, and to larger capacity generators where sets of two or more nozzles are placed in parallel to obtain a greater vacuum flow rate.
Thus, what is sought is a manner of attenuating or damping the amplitude of vacuum pressure oscillations in a vacuum system, and more particularly, which is self modulating to attenuate or damp high amplitude vacuum pressure oscillations to limit or prevent transmission thereof to sensitive apparatus connected to the system such as instruments and tools.